As Stephen Hawking is promoting his new series on the Discovery channel, he has also given an interview to the Times of London in which ‘he has suggested that extraterrestrials are almost certain to exist but that instead of seeking them out, humanity should be doing all that it can to avoid any contact.’ Hawking says, ‘I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach. … If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.‘
http://www.timesonline.co.uk/tol/news/science/space/article7107207.ece
Is hiding under our collective bed here on Earth an option for humanity in the wider galaxy?
The whole concept is one of paranoia. Considering the age of the galaxy, advanced species could be of immense ages. Any new interstellar aggressor species would find itself confronted by a whole range of progressively more advanced species each in turn more capable of deploying more advanced and often more subtle forms of social stabilization. The simplest method by which to judge species and what measures may be required to control threats implied by them, is the way in which they interact with less advanced species.
Much the same way a species upon it’s own planet would be judged by the way they interact with each other, with suppositions of racial differences where none exist, of artificial regional divides, specifically demonstrated where a species of one region preys upon and exploits the same species in another region, with claims of racial differences to hide or degenerative social diseases, like psychopathy and narcissism.
Most likely any threatening species would be dealt with, likely well before they became destructive upon an interstellar basis. The greater the gap in advancement the less likely communication will occur, as there will always be more similarly advanced species to fill that interaction and monitoring gap, who in turn would be monitored by next nearest level of amelioration.
Besides, in reality planets are pretty crappy resources for any interstellar species, nebula and dust clouds have stupendously huge quantities of material available, sufficient to make thousands even millions of suns, already in affect mined, granulated to a fine powder and just requiring filtering to extract the desired elements.
Humanity has to be far more concerned with how they interact with each other and how that interaction could be interpreted from an external viewpoint and whether it could be considered as potentially threatening and what actions are required to nip the threat in the bud. Whether it be social modification and, or culling of specific socially destructive elements.
Of course it is interesting to note that Carl Sagan pointed out that the first high-powered TV broadcasts that might be intercepted by the aliens show Hitler at the Nürnberg party rally. 
